The present invention relates to electromagnetic wave reception and transmission apparatus, and more particularly to a reflector type antenna having one or more mechanical assemblies disposed on the reflective surface thereof for achieving high quality adaptive nulling throughout the sidelobe region of the antenna.
Various techniques have been used in the past for nulling or suppressing selected sidelobes in directional antenna systems. One such technique which is particularly suitable for phased array systems involves the electronic control or measurement of the individual element signals of the array to ultimately produce a null or nulls in the antenna pattern. This form of adaptive nulling, however, requires costly components and complex algorithms, often requires long signal processing times, and is not usually suitable for reflector type antennas.
Another known adaptive nulling technique for reducing sidelobe interference in reflector antennas and phased arrays is called sidelobe cancellation. It involves combining the signal from the main antenna with a signal derived from an auxiliary antenna, which is suitably adjusted in amplitude and phase. Such auxiliary antenna systems, however, require high precision components and complex microwave circuitry to achieve adequate system performance.
In U.S. Pat. No. 4,376,940 issued to H. Miedema on Mar. 15, 1983, there is disclosed an antenna arrangement for suppressing sidelobes by mechanical means, in which the arrangement consists of a pair of auxiliary antennas located at the edges of the reflector antenna such that, when the energy from the main and auxiliary antennas are combined in a hybrid coupler and associated circuitry, certain sidelobes are suppressed. An alternative embodiment of the invention involves laterally offsetting edge segments of the reflector to produce a radiation pattern which is phased opposite to the selected sidelobes to be suppressed.
A problem associated with sidelobe nulling by adjusting the edge segments of a reflector is that the energy reflected from the vicinity of the edge or rim of the reflector to the feed horn is purposely made much lower than the energy from the center region of the reflector to the feed horn. This is done in order to reduce overall sidelobes. Therefore such edge-located antenna adjusting techniques require relatively large reflecting areas to provide a signal of adequate amplitude for nulling. Also, once the physical size of the auxiliary segments has been selected, only sidelobes whose gains are nearly equal to the gain of the auxiliary segments can be nulled effectively. It is also apparent that retrofitting existing antennas to include auxiliary antennas or adjustable edge reflectors or segments could be prohibitively expensive.
The concept and practicality of adaptive nulling in a reflector antenna by means of at least two small and adjustable reflective disks disposed on the focusing surface of the antenna is disclosed and claimed in U.S. Pat. No. 4,631,547 of Daniel Jacavanco, issued Dec. 23, 1986 and entitled "Reflector Antenna Having Sidelobe Suppression Elements". This concept is also disclosed by Mr. Jacavanco in his paper entitled "Controlled Surface Distortion Effects", which appears in the Proceedings of 1984 Antenna Applications Symposium, September 1984. A signal is produced at the antenna feed output which is a function of the energy reflected by the disks as well as that reflected by the reflector. By adjusting the spacing between the disks and the reflector surface, the phase and amplitude of the resulting nulling signal can be varied relative to the antenna signal. This technique allows intentional or unintentional sources of interference to be nulled or eliminated. However, two interacting disks are needed to achieve amplitude as well as phase control.
Nulling with only one disk can only be achieved in those regions where the sidelobe signal is within one or two decibels of the signal level provided by the single disk. If the sidelobe signal from the antenna and the signal from the single disk are not approximately equal, then to achieve nulling, the size of the disk must be increased or decreased.